Molding of various plastic materials is a common method used to create parts for various applications. One specific method of producing parts in this manner is through the use of a sheet molding compound (SMC). Several examples of SMC methods are Long Fiber Injection (LFI), Structural Reaction Injection Molding (SRIM), Reinforced Reaction Injection Molding (RRIM), or Vacuum Assisted Resin Transfer Molding (VARTM).
The method of making a part using LFI involves the use of fiberglass cut into long strands, or fibers, mixed with a resin to provide additional strength to a molded part. SRIM molding is similar to typical injection molding, except that thermosetting polymers are used, and the two parts of the polymer are mixed together an injected into a mold under high pressure, and are then allowed to cure. A reinforcing agent is used, such as glass fillers, in the mixture for improved strength. RRIM is another type of SRIM, in which a fiber mesh is used as the reinforcing agent; the fiber mesh is placed in the mold and the polymer mixture is then injection molded over it. VARTM is a process in which resin is inserted into a mold which contains layers of fibers or a preform; flow of resin is assisted by a vacuum in the mold.
There has been an increasing desire to create parts that are not completely solid, but rather have a type of insert in between layers of SMC. Having a part which is not completely solid is less costly, and is typically lighter in weight, while more effectively using the strength of the SMC.
While each of the methods described above has different advantages, for example, such as different types of resins may be used, they have also been met with various drawbacks in that each one of the above-mentioned processes requires additional manufacturing steps, increased tooling, and increased difficulty of assembly. Additionally, incorporating one of the above-mentioned manufacturing processes while using an insert to reduce the amount of SMC necessary requires the use of a heavy insert because a lightweight insert becomes crushed during the molding process.
Accordingly, there exists a need for a method of producing parts by molding an SMC over an insert or core which is lightweight and can withstand the various pressures and temperatures of molding, while maintaining a high strength to weight ratio.